Meanwhile I did find, that user/template-tags.php did have a known issue. But patching that did not change behaviour at a specific way to reproduce the problem:
User X is member of group A, but not group B. Both groups do have hidden forums. Both Forums are in a hidden Category “Gruppenforen” (German for group forums).
When User X goes to the forum of group A and then follows the link in the group to “Gruppenforen” and onlky at that moment the widget for recent topics shows all topics in “Gruppenforen” includigts those of group B, that user X is not supposed to see.
